There are several types of retroreflective sheeting including exposed lens, embedded or enclosed lens, and encapsulated lens retroreflective sheeting. In general, retroreflective sheeting comprises a multiplicity of lens elements and a reflector located behind the lens elements. Lens elements may be, for example, glass or ceramic microspheres, or microprisms in a polymeric base sheet. The reflector may be a layer of reflective metal such as aluminum which has been vapor-deposited on the lens elements or at some other location behind the lens elements, or flakes of reflective metal embedded in a binder layer, etc.
Exposed lens retroreflective sheeting generally comprises a polymeric binder film in which a multiplicity of lens elements such as glass microspheres are partially embedded, and a reflecting means is disposed on the backside of the embedded lens elements. Embedded or enclosed lens retroreflective sheeting generally comprises glass or ceramic beads completely embedded in a polymer binder film or multiplicity of films. A mirror or reflective surface, generally formed from a metallic vapor coat is formed on the backside of the polymer/bead composite. In a typical operation, light passes through the beads which individually act as lenses focusing the light and directing the light against the mirrored surface. The light is then reflected back through the beads and toward the source. Typically, the mirror surface is separated from the glass beads by a spacing layer or "space coat" which provides the desired focal length between the beads and the reflective surface. Thus, the elements of a typical enclosed or embedded lens retroreflective sheeting are: lens arrangement (beads embedded in polymer), spacing layer (space coat), and reflector surface (vapor coat). The sheeting may include other elements such as a transparent covering layer conforming to the front extremities of the lens elements and having a flat front face, and an optional transparent top layer for improving weatherability.
Encapsulated-lens retroreflective sheeting generally is formed from (1) a base sheet comprising retroreflective elements (glass beads) partially embedded in a binder layer and (2) a cover sheet. The cover sheet is sealed to the base sheet along a network of interconnecting bonds to form hermetically sealed cells containing protected retroreflective elements having an air-interface between the glass beads and the cover sheet.
Retroreflective sheeting has many applications. For example, enclosed lens retroreflective sheeting may be applied to a substrate such as wood, plastic or metal, and used to form highway signs, license plates, safety signs and reflectors. In many of these applications, depending upon the application of the retroreflective sheet, the sheeting should be characterized by one or more of the following properties: reflectivity, embossability, printability, flexibility, etc.
A number of polymer resins have been suggested as useful in forming the transparent cover film and protective topcoat on retroreflective sheeting. When the retroreflective sheeting is to be formed such as by embossing, the protective topcoat is generally a thermoplastic resin. Plasticized polyvinyl chloride resins, for example, have been used as transparent topcoats. U.S. Pat. Nos. 4,767,659 and 4,664,966 describe thermoplastic polymer topcoats selected from (a) one or more aliphatic urethanes, (b) one or more copolymers of monomers comprising by weight a major proportion of at least one of ethylene and propylene and a minor proportion of another monomer, and (c) homopolymers of ethylene or propylene. A preferred aliphatic urethane described in this patent is prepared from an aliphatic polyfunctional isocyanate and a polyfunctional hydroxyl-containing polymer. U.S. Pat. Nos. 4,505,967 and 4,950,525 describe elastomeric retroreflective sheeting wherein the cover layer may comprise a thermoplastic elastomeric aliphatic polyurethane. U.S. Pat. No. 4,808,471 describes a flat transparent topcoat for retroreflective sheeting which is as tough and as resistant to solvent and abrasion as alkyd resins and is weather-resistant like acrylic polyesters. The topcoat described in the '471 patent is made from a mixture of a hydroxy-functional acrylic polyol and a curing agent which is an aliphatic polyfunctional isocyanate.
In U.S. Pat. Nos. 4,755,425, 4,844,976 and 5,073,404, the patentee describes retroreflective sheeting which is coated with a mixture of silica in a transparent polymer selected from the group consisting of aliphatic polyurethanes and polyvinyl chloride copolymers having a minor amount of a comonomer containing at least one carboxylic acid or hydroxyl moiety. The silica comprises from 10 to 80 weight percent of the transparent protective coating. The silica-containing coating is described as providing superior soil and dew-repellency and is reported to retain a higher percentage of original brightness after exposure to dirt and/or dew conditions.
U.S. Pat. No. 5,117,304 describes flexible retroreflective articles based upon an optically clear aliphatic polyurethane polymer containing a plurality of hard chain segments. These polyurethanes are used in the formation of the retroreflective elements of the article.
U.S. Pat. No. 5,514,441 describes retroreflective sheeting of the enclosed type having an improved topcoat layer or cover sheet which is an aliphatic polyurethane. The aliphatic polyurethane topcoat is obtainable from a mixture comprising (A) an aliphatic polyurethane derived from an aliphatic polyisocyanate and a polyester polyol and (B) cellulose acetate butyrate. The mixture may also contain (C) at least one non-reactive solvent, and, optionally, (D) one or more surfactants including, for example, hydroxy-containing non-silicone surfactants such as hydroxyalkyl imidazoline surfactants.